feat(Algebra/FreeMonoid): FreeMonoid.length is surjective

[plp127]

Prove the theorem FreeMonoid.length_surjective, stating that FreeMonoid.length is surjective when α is nonempty.

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[github-actions]

PR summary 6c6b8f987a

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff (regex)

+ length_surjective

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit 6c6b8f9).

• +2 new declarations
• −0 removed declarations

+FreeAddMonoid.length_surjective
+FreeMonoid.length_surjective

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No changes to strong technical debt.

No changes to weak technical debt.

Current commit 6c6b8f987a
Reference commit c81c5cbb1c

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).